Phase locked loops are used in various applications and particularly in communication devices to produce output signals at a stable frequency. To this end, a phase locked loop contains a frequency controllable and voltage controlled oscillator whose output signal connection is usually connected to a frequency divider. The frequency divider divides the output signal using a particular division ratio and supplies the divided signal to a phase detector. The phase detector compares the phase of the divided signal with the frequency of a reference signal and produces a regulating signal therefrom in order to set the output frequency of the voltage controlled oscillator. The regulating signal is supplied via a loop filter to the voltage controlled oscillator, which uses it to readjust its output frequency and thus to compensate for the discrepancy.
To obtain a plurality of different output frequencies from the phase locked loop, it is appropriate to implement the frequency divider with an adjustable division ratio. Such a frequency divider is also called a multimodulus divider. The division ratio of the multimodulus divider is set using an actuating signal. The document “A Pipelined Noise Shaping Coder for Fractional-N Frequency Synthesis”, IEEE Transactions on Instrumentation and Measurement, Vol. 50, No. 5, October 2001 by Mücahit Kozak et al presents a phase locked loop with a frequency divider whose division ratio is set using a MASH modulator (multistage noise shaping modulator). A MASH modulator is a specific embodiment of a delta-sigma (Δ-Σ) modulator and is particularly suitable for providing very fine-resolution actuating signals for the purpose of setting the division ratio of the frequency divider.
Frequency dividers whose division ratio can thus be set very quickly using a MASH modulator are preferably used in phase locked loops.
The freewheeling frequency of a voltage controlled oscillator and also the loop gain of phase locked loops have an unavoidable drift. The reason for this is the inherent heating of the voltage controlled oscillator or a drift in reference currents or reference voltages within the control loop. This drift, also referred to as phase transient, results in a slow change in the phase of the output signal. The integral relationship between phase and frequency means that a phase drift over this time also produces a frequency offset in the output signal from the oscillator.
The brief frequency error and also the phase transient are largely corrected in the phase control loop using a loop filter which is characterized by an integrating response. In this context, a phase locked loop with an integrating loop filter is referred to as a type II phase locked loop. If a loop filter with a nonintegrating response is used in the phase locked loop instead of an integrating loop filter, the brief frequency error or the phase drift becomes markedly more noticeable.
Particularly in the case of modulation methods for telecommunication standards in which the information is held in the phase, the phase drift of a phase locked loop is superimposed on a phase modulation and thus corrupts the actual useful signals. Examples of modulation methods with phase modulation are FSK (Frequency Shift Keying) or PSK (Phase Shift Keying) modulations, such as are used in the GSM mobile radio standard. OFDM (Orthogonal Frequency Division Multiplexing) modulation is also particularly sensitive to changes in the phase.
However, a loop filter with a nonintegrating response which is connected between the output of the phase detector and the actuating input of the voltage controlled oscillator has the advantage of a flat group delay time. This allows low-distortion transmission of the regulating signals and likewise permits direct frequency modulation of the output signal from the control loop with a useful signal. In addition, a nonintegrating loop filter is insensitive toward leakage currents within the charge pump and also has a greater degree of linearity between the signal which is output by the phase detector and the actuating signal which is output by the loop filter to the voltage controlled oscillator. The use of a nonintegrating loop filter therefore permits much higher transmission rates for particular modulation methods.
An example of such a drift in a voltage controlled oscillator in a phase locked loop is shown in FIG. 5. In this case, the phase locked loop contains a nonintegrating loop filter (type I phase locked loop). The time is plotted along the x axis, and in the present measuring range is 600 μs. The y axis shows the phase in relation to a reference phase. The figure clearly shows the exponentially decreasing profile of the phase drift over the measured time.